Lucca, Dicembre 2013 1 Codes for existing structures Pietro CROCE Dipartimento di Ingegneria Civile e Industriale Università di Pisa Project number: CZ/011/LLP-LdV/TOI/134005 Seminar: Assessment of existing structures • Need and criteria for codes and recommendations • Example codes • Example contents with illustrations • Safety acceptance – performance criteria • Applicability to case studies • Future tendencies
This document is posted to help you gain knowledge. Please leave a comment to let me know what you think about it! Share it to your friends and learn new things together.
Transcript
Lucca, Dicembre 20131
Codes for existing structuresPietro CROCE
Dipartimento di Ingegneria Civile e IndustrialeUniversità di Pisa
Project number: CZ/011/LLP-LdV/TOI/134005
Seminar: Assessment of existing structures
• Need and criteria for codes and recommendations• Example codes• Example contents with illustrations• Safety acceptance – performance criteria• Applicability to case studies• Future tendencies
Why reassess an existing structure?
• Deviations from original design• Doubts about safety• Adverse inspection results• Change of use• Lifetime prolongation• Inadequate serviceability
Structural failures experience
Typical questions
• What type of inspections are necessary?• What type of measurements shall be
taken?• What analyses shall be performed?• What is the future
risk in usingthe structure?
How to find the Answers
• No classical code approach• New information becomes available• New techniques can be implemented• New material technologies can be used• New decision criteria under new
uncertainties
Questions related to codes
• Are existing structures covered by codes for new structures?
• Is there a separate code and to which type of buildings does it apply?
• Do codes allow for relaxation or lower performance?
• What aspects are covered (inspections etc.)?
• What are the governmental regulatory bodies behind?
Possible requirements for a code on existing structures
•Applicability : the code should be applicable to typicalassessment cases.
• Compatibility to codes for new structures: the codeshould use the same philosophy as current codes fornewstructures.
• Flexibility : the code should be flexible to includeadditional information gained by inspection.
• Ease of use: the code should be understandable toengineers and easy to use in practice.
• General Framework of Assessment• Data for assessment• Structural Analysis• Verification (Limit State)• Assessment based on satisfactory past performance• Interventions• Report• Judgement and Decisions
ISO 13822
General flow of assessment
Procedures
Phase: Preliminary Assessment
• Visual inspection
• Review of documentation
• Code compatibility
• Scoring system:1. age of the structure2. general condition3. loading (modifications)4. structural system5. residual working life
Phase: Detailed assessment
• Additional inspections
• More detailed analyses1. progressive collapse2. full probabilistic3. sensitivity analyses4. risk analyses
Phase: Detailed Assessment
• Quantitative inspections
• Updating of information
• Structural reanalysis
• Reliability analysis
• Acceptance criteria
Histogram
0
5
10
15
20
25
30
35
0 30 42 54 66 78 90 102
Freq
uenc
y
Frequency
Normal
Lognorm "0"
Gumbel
Lognormal
Gamma
New Information (Updating)
A) Proof Load
B) Variables (concrete strength)
A) Example: Proof Loading (Survival of a load)> Updating of resistance
B) Example: Concrete strength data
Histogram
0
5
10
15
20
25
30
35
0 30 42 54 66 78 90 102
Fre
quen
cy
Frequency
Normal
Lognorm "0"
Gumbel
Lognormal
Gamma
Decision Criteria
• Target reliability• Economical considerations• Time constraints• Sociopolotical aspects• Codes and standards• Complexity of analysis• Experience in other fields
Safety Acceptance Criteria
- European Experience (limit state verification)
- New practice in the US (performance based design)
- Optimisation based on LQI- Judgement
Conclusions regarding reliability acceptance
• A lower safety level compared to a new structure is acceptable
• Various criteria have been proposed in the literature
• Acceptance criteria depend on cost of safety, consequences of failure, desired residual lifetime
• A decrease of the acceptable reliability index ß by 0.5 can be recommended
Example: Updated earthquake acceleration
ID 39892
Latitudine 39,30°Longitudine 16,27°
Seismic hazard curve and updated value due to relaxed acceptance criteria
Mu: Ultimate Bending MomentMa: Acting Bending Moment
Variable Distribution c.o.v.
Steel strength
Lognormal 0.06
Concrete Strength
Lognormal 0.14
Cover thickness
Lognormal 0.25
Updating of random variables(due to destructive tests)
Reliability index ß is increased from 3.70(prior information) to 3.80, due to
reduced variability of the parameters
Typical limit states
- extreme load
- Fatigue
Which measures are necessary in order to meet acceptance criteria (residual life time 20 years)?
Steel road bridges
(Phase 3 Procedure)
Fatigue models
• Fracture Mechanics approach• Crack growth propagation• Influence of inspections (measurement of
cracks)
2ca
Wf
A
Bb
2c
Sr
Sr
Wf
b
WcpL
θ
25m 30m 25m
5m
Detail locationCover plate detail
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
0 0.5 1 1.5 2 2.5 3 3.5
Crack size (mm)
DetectionProbability
ECT
DPI
CWMPI
ACFM
Variable Distribution Type
ad POD* Inspection
ag Uniform Repair
afail Derived Mixed
Sr RayleighLoad
Smax Gumbel
Fatigue assessment: Random Variables (examples)
* POD for MPI used in case study
1,00E-05
1,00E-04
1,00E-03
1,00E-02
1,00E-01
1,00E+00
30 60 90 120T (Years)
Pf
GIADI =∩=
LTIADI =∩=
GLTIADI +=∩=
DI =
Prior
I: Inspection, D=DetectionIA: Invasive Action, LT=Load Truncation, G=Weld Toe Grinding
Fatigue assessment: typical results
• Inspection and crack detection at T=30y• Alternatives considered:
1. Load truncation (LT)2. Weld toe grinding (G)3. Load truncation + weld toe grinding (LT+G)
Fatigue assessment: scenarios
Future tendencies
• No classical code approach• Quantification of new information • Updated design values• Uncertainties (climate change etc.)• Relaxed acceptance criteria• Robustness aspects
Vita nominale di progettoTabella 2.4.I – Vita nominale VN per diversi tipi di opere
TIPO DESCRIZIONE Vita Nominale
VN
(in anni) 1 Opere provvisorie – Opere provvisionali – Strutture in fase costruttiva(1)
≤ 10
2 Opere ordinarie, ponti, opere infrastrutturali e dighe, di dimensioni contenute, o di importanza normale
≥ 50
3 Opere, ponti, opere infrastrutturali e dighe, di grandi dimensioni, o di rilevante importanza
≥ 100
1] Le verifiche sismiche di opere provvisorie o strutture in fase costruttiva possono omettersi quando le relative
durate previste in progetto siano inferiori a 2 anni.
Tab. 2.4.I – Valori minimi della Vita nominale VN di progetto per i diversi tipi di costruzioni
TIPI DI COSTRUZIONI DI NUOVA REALIZZAZIONE
Valori minimi
di VN (anni)
1 Costruzioni temporanee(1) (provvisorie) 10
2 Costruzioni con livelli di prestazioni ordinari 50
3 Costruzioni con livelli di prestazioni elevati 100
(1) Costruzioni o parti di esse che possono essere smantellate con l’intento di essere riutilizzate non sono da
considerarsi temporanee
NTC 2008
Vita nominale di progettoLa vita nominale di progetto VN di un’opera è convenzionalmente definita come il numero di anni nel quale l’opera, purché soggetta alla manutenzione ordinaria così come prevista in sede di progetto, è previsto che mantenga i livelli
prestazionali per i quali è stata progettata.Non è una vita intesa in senso biologico